The present invention is directed generally to prestressed concrete tanks used for storing liquids such as water and, more particularly, to such prestressed concrete tanks having means for preventing the wall of the tank from rotating and/or from shifting laterally relative to the tank foundation when subjected to shearing forces, such as occur during backfilling earthquakes, and the like.
As a brief background, conventional prestressed concrete tanks generally comprise an annular-shaped supporting foundation embedded in the earth and a vertically extending cylindrical wall structure of concrete about a cylindrical steel shell. The outer concrete portion of the wall structure is generally prestressed by a plurality of circumferential steel wires under a tension of up to about 140,000 psi or more. Prestressing of the wall structure of the tank maintains the concrete in compression even when the tank is filled with a liquid. The wall structure of the tank is not necessarily fixed to the foundation, but, may instead simply rest on the foundation so that the wall structure is able to move or float horizontally with respect to the foundation. An elastomeric gasket type seal (e.g. PVC) is utilized between the wall structure and the foundation to prevent liquid contained in the tank from escaping between the bottom of the wall structure and the foundation.
The capability for limited movement of the wall structure relative to the foundation is necessary during the construction of the tank and during its service life since, as the prestressing circumferential steel wires are stressed, the radius of the wall structure is reduced as well as when the concrete material of the wall structure cures. Furthermore, when the completed tank is filled with or simply contains a liquid, hydrostatic pressure of the liquid causes a radial expansion of the wall structure. In both instances, the wall structure must move relative to the foundation of the tank to prevent harmful stresses from being created, both in the wall structure and the foundation.
However, a capability of almost unlimited movement of the wall structure relative to the foundation is undesirable and even catastrophic when the wall structure is subjected to large horizontal shearing forces. An example of the occurrence of such shearing forces is during the backfilling of the tank. To be more specific, in many instances the terrain on which a tank is to be constructed is not level but is instead sloped, e.g., on the side of a hill. Consequently, prior to construction of the tank, earth is excavated from the hillside to provide a level base for the tank. Once the tank is completed, earth is backfilled along the uphill side of the wall structure of the tank to insure adequate drainage and to prevent erosion about the foundation of the tank. Backfilling on only a portion of the wall structure, however, creates shearing forces which tend to displace the wall structure off the foundation in the downhill direction, or at least tend to cause the wall structure to slide across the foundation. Another example of the occurrence of such forces is during an earthquake or earth tremor wher unequal shearing forces on the tank can be expected from any direction and which will tend to displace the wall structure.
Consequently, it is oftentimes necessary to provide means in a concrete tank which can help resist the occurrence of these shearing forces and thereby help prevent large displacements of the wall structure of the tank relative to the foundation while still allowing for necessary limited movement of the wall structure during construction of the tank and upon filling the completed tank with liquid.
An example of a known prestressed concrete tank which does include such means is disclosed in U.S. Pat. No. 2,932,964 to Dobell. In this tank, an overlap between an elevated peripheral step on the foundation and a protrusion on the interior of the wall structure provides a key to assure that the tank will stay on its foundation during earthquakes and to maintain the tank in position when the tank is backfilled unevenly and movement occurs. However, means are not provided in the disclosed tank to prevent rotational displacement as well as lateral displacement of the wall structure relative to the foundation.
Furthermore, U.S. Pat. No. 3,233,376 to Naillon et al discloses structural shear units adapted for use in concrete tanks. These units are devices which provide a connection between a supporting structure such as a tank foundation and a supported structure such as a tank wall, and which permit relative motion due to loading, prestressing or seismic stress. The disclosed structural shear units each comprise a lower member and an upper member slidably supported on the lower member, the members being respectively secured to the supporting structure and to the supported structure. The members have mutually engageable elements such as pairs of laterally directed abutments extending horizontally generally along the axis of the desired free sliding movement for restraining or limiting horizontal sliding motion between the members in the direction perpendicular to the axis. To reduce friction, an anti-friction pad such as a sheet of porous metal inpregnated with a lubricant is placed between the members.